Electron multiplier tube



' p 5, 1939- R. L. SNYDER 2,172,155

ELECTRON MULTIPLIER TUBE Filed Nov. 29, 1957 /3 35 I le/VH5.

I lamlllllllhllullmlflllmw Patented Sept. 5, 1939 mesne assignments, to Farnsworth Television & Radio Corporation, Dover, Del., a corporation of Delaware Application November 29, 1937, Serial No. 177,065

5 Claims.

My invention relates to electron multiplier tubes, and more particularly to electron multiplier tubes of the so-called d-c type, wherein multiple stages of secondary electron emitting material are energized at successively increasing steady potentials.

Among the objects of my invention are:

To provide an electron multiplier wherein space charge limitations are greatly reduced; to provide a high powered electron multiplier tube; to provide an electron multiplier wherein guiding fields are produced by the shape of the electrodes; to provide an electron multiplier tube utilizing a Faraday space; to provide an electron multiplier tube utilizing a modified Faraday cage; to provide an electron multiplier having a cone-shaped emitter, together with means for guiding electrons; and to provide asimple and high powered electron multiplier tube.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claims.

The figure shows a preferred type of electron multiplier tube built according to my invention, together with one circuit by which the tube may be energized.

My invention may best be understood by direct reference to the drawing, wherein an envelope I is provided at one end witha reentrant stem 2 through which multiple leads passand supports are anchored, carrying the main body of the multiplier structure. At the opposite end of the tube is a second reentrant stem 3 carrying an output anode 4.

Beginning at the axis of the reentrant stem 2, heater leads 5 support a filamentary heater 6, energized by heater source I. Surrounding the heater is a unipotential cathode 9 having its own lead It, the latter being connected, in the particular circuit shown, to ground and to the negative end of a steady potential source I3.

Immediately surrounding the unipotential cathode is a control grid II, energized by any convenient input circuit 12 through control grid lead I4. Surrounding the control grid II and concentric therewith is a cylindrical first stage screen l5, attached to the bottom of an annular cup 16 which constitutes the first secondary emitting stage of the multiplier, and this annular cup is made ready to emit secondary electrons by any of the well known processes for creating secondary emission at a high ratio, such as, for example, making the cup of silver, oxidizing it, and then sensitizing with caesium. The same treatment will also serve to sensitize other stages, as will be described later.

Immediately surrounding the annular cup l6, and at a level with the rim of the cup and spaced therefrom, is an annular disc l1, likewise of silver and sensitized for a secondary emission, and constituting the third stage. Immediately above the unipotential cathode-grid structure is a cone it, the apex of which is axially located and pointing toward the cathode-grid structure, and this cone constitutes the second secondary emission stage. The cone i9 is supported from stem 2 by support wires 20, one of which is prolonged through the stem into lead 2|, and third stage I! is similarly supported on support wires 22, one of which is prolonged through the stem into lead 23.

A cylinder 24 surrounds the space between annular cup l6, the third stage 11, and the cone stage 19, this cylinder being preferably made of sheet material and provided with apertures 25, so that caesium vapors may readily enter the inside thereof. This cylinder, which constitutes part of a Faraday cage, is not sensitized for secondary emission, and therefore may be made of such material as nickel, tantalum, molybdenum, etc. The bottom of cylinder 24 is closed by a lower'screen 26, extending across the tube parallel to third stage I7 and slightly above it. This lower screen 26 is provided with a central aperture 21, and immediately below this aperture is a cathode disc 29 which is attached to unipotential cathode 9, and therefore at ground potential. An upper screen 30 closes the top of the cylinder, this screen extending parallel to the lower screen until it reaches the neighborhood of the cone I9, and then the upper screen 30 follows the contour of cone i9, but spaced slightly from it. Anode 4 is placed immediately above the cone l9 and upper screen 30.

Annular cup I6, constituting the first stage, is attached by its lead 31 to source l3, at a potential just above cathode 9. The second stage, which is the cone I9, is attached at the next highest potential through lead 2|. The annular disc ll, constituting the third stage, is attached to source I3 at the next highest potential through lead 23, and the Faraday cage, comprising cylinder 24 and upper and lower screens 26 and 30, respectively, is attached to the next highest potential through lead 32, and anode 4 is attached to source l3 at the highest potential through output resistor 35, and the output may be taken from any convenient output lead 36.

In operation, when all stages are energized, heater 6 raises the temperature of unipotential cathode 9 so that electrons are emitted therefrom. 'Ihese electrons are accelerated by screen cylinder i5 and passed therethrough to impact the inside of annular cup It at a velocity suflicient to create secondary electrons at a ratio greater than unity. The space within the annular cup is exposed only to the potential of lower screen 26, so that the secondary electrons emitted from the first stage 36 are drawn through the lower screen 26 into the Faraday space enclosed by the Faraday cage, and are drawn upwardly to impact the sides of cone l9 which, being sensitized for secondary emission, causes the creation of new secondaries upon impact therewith. These new secondaries are accelerated perpendicular to the conical surface because of the close proximity of upper screen 38, and are directed along this perpendicular and will thereafter hit third stage H, where again new secondaries are created which will be accelerated away from the surface along a perpendicular because of the positioning of lower screen 25. These la'stsecondaries will again ass'through the equipotential space of the cage, pass through the upper screen 30 between the edges of the cone and the cylinder 24, and be drawn to anode 4. Thus, I have provided a three stage amplifier, with the electrons passing three times through the Faraday cage.

In a high powered tube such as would be used in transmitters, current outputs are needed up to one or two amperes. The present type of tube will give such outputs, because with the exception of the first stage, which operates at relatively low current levels, the accelerating screens are very close to the secondary emitting electrodes, elim- .i ating the possibility of space charge in front of these electrodes. Number one stage, the annular cup E6,has a strong field above it, so that with its low current it is unlikely that any space charge limitation will take place at this point.

I have preferred, however, to modify the field within the Faraday cage along the axis of the tube, and for this reason I have provided aperture 2'! 'in the lower screen 26, and positioned immediately below this aperture a disc 29 attached' to the cathodewhich is at the lowest potential. This disc projects a strong'negative field to the apex of the cone, and directs electrons away from the center portion of the cone, thus ensuring the fact that the electrons land on the sides of the cone. It has been found that the use of this strong field within the Faraday cage prevents electrons crossing in the middle of the space and approaching the second stage at an unfavorable angle for impact, and in effect alines the electrons admitted from annular cup l6 into a cylindrical beam and focuses them properly upon the sides of cone l9. Obviously, it is highly desirable that the electrons from the first stage fall on the cone-shaped second stage in such areas that when the secondaries are emitted therefrom that they can be reaccelerated along a perpendicular which will intersect the third stage I7. v I

I have thus provided an electron multiplier tube which can be utilized at high power without detrimental space charge limitations, and of course it should be clearly understood that this tube may be used in any circuit known to those skilled in the art, and that the circuit shown herewith is purely ilustrative.

I claim:

1. An electron multiplier tube comprising an envelope containing a source of electrons, an annular cup surrounding said source and presenting an inner surface, capable of emitting secondary electrons at a ratio greater than unity upon electron impact therewith, to said source, said cup having an electron permeable portion surrounding said source, a cone-shaped secondary emissive electrode having its apex presented to the open end of said cup and coaxial therewith, a third secondary emissive electrode adjacent said cup and intersecting perpendiculars erected from the surface of said cone, a Faraday cage between said cone and said cup and said third electrode, and having electron permeable portions adjacent and parallel to the surface of said cone and of said third electrode, means for controlling electrons emitted from said source, and electron collecting means without said cage in the path of perpendiculars erected from said third electrode.

2. An electron multiplier in accordance with claim 1 with electrode means for modifying the field within the Faraday cage between said source and the apenx of said cone.

3.'An electron multiplier in accordance with claim 1 with an aperture in said cage just above said source and a field modifying electrode connected to said source and positioned between said source and said aperture.

4. An electron multiplier in accordance with claim 1 where said third electrode is an apertured disc placed at the level of the cup opening and surrounding it.

5.,An electron multiplier in accordance with claim. 1 where the Faraday cage has a peripheral cylindrical wall provided with large apertures to promote gaseous circulation between the interior of said cage and the exterior thereof.

RICHARD L. SNYDER. 

